Ranging system

ABSTRACT

A radiated-signal reflector ranging system comprising a time measuring device which is started upon the transmission of a signal, and a plurality of registers connected over gates and read-out lines to the time measuring device. The gates are normally closed and are momentarily opened in turn in response to incoming reflected signals, thereby causing the times of arrival of the reflected signals to be recorded in the registers.

United States Patent 1 [111 3,743,419 Skagerlund July 3, 1973 [5 RANGINGSYSTEM 3.503.680 3/1970 Schenkerman 356/5 [75] Inventor: Lars-ErikSkagerlund, Karlskoga, FOREIGN PATENTS OR APPLICATIONS Swede 957,2355/1964 Great Britain .356/4 [73] Assignee: Aktlebolaget Bofors, Bofors,Sweden Primary ExaminerBenjamin A. Borchelt 22 F1 d. l 9,1969 1 [e yASSISIHHI Exammer-S. C. Buczmski [21] Appl. No.: 840,276 AttorneyHane &Baxley [52] US. Cl. 356/5, 343/13, 343/5 DP [57] ABSTRACT [51] Int. ClG01c 3/08 A radiated-signal reflector ranging system comprising [58]Field of Search 356/4, 5, 343535)? a time measuring device which isstarted upon the transmission of a signal, and a plurality of registerscon- 56] R f d nected over gates and read-out lines to the time meaeerences suring device. The gates are normally closed and are UNITEDSTATES PATENTS momentarily opened in turn in response to incoming3,153,230 10/1964 Krevelen et al 343/13 reflected signals, therebycausing the times of arrival of 33%? c j ---3 4 3 //6 2 the reflectedsignals to be recorded in the registers. 3,4 1 0x, r. eta. 3,545,86112/1970 Farnsworth et a] 356/5 7 Claims, 2 Drawing Figures 'Q/M/JM fft?T 5 COUNTER T/Mf DELAY 1 /4 I l6 I /5 9 4 5 505/ w/va MFA/V5 E) awe/(iGATE 'K/ 7 3 GA was 5 4 COUNTER T)7 a /9 DIS TR/BU TOR /2 /3 42 ,4; 40GATE GATE GATkldo A: 1 Lil- 2/ Z2 23 k SW/TCH mes/v19? (REGISTERREG/STER PMENTEBJM a ma SEEIIUZ kCOU/VTER T/ME DELAY -/8 KEcEIV/A/aMEANS 2 w awe/( GAT 7 GA 7':

K 5 COUNTER) 7 f A /9 DISTRIBUTOR GATE GATE L 20 I 2/ 22 23 SMI/TCH(REGISTER [REGISTER (REGISTER FIG. I

l N VEN TOR.

LHRS- 81K f/(RGERL (MID BY MM RANGING SYSTEM BACKGROUND OF THE INVENTIONThe invention relates to improvements in a ranging system of the kindcomprising a transmitter for the transmission of a brief pulse ofelectromagnetic radiation, e.g. a light pulse generated by a laser, areceiver for the reception of reflected pulses, and timemeasuring meansfor measuring the time interval between the transmission of a pulse andthe reception of a reflected pulse. This time interval is a measure ofthe distance to the object which reflected the pulse, and the rangingsystem is provided with means which make possible a direct readout ofthe distance for instance from a digital'display unit.

The time measuring means comprises an oscillator which generateselectric oscillations, and a counter which counts the oscillationsgenerated by the oscillator from the time of the transmission of apulse.

In the measurement of distances with a system of this kind andparticularly when a laser transmitter is used it often happens that aplurality of echo pulses are received for each transmitted pulse becausethe transmitted radiation is reflected not only by the object to whichthe distance is to be measured, but also by other objects located beforeand beyond this object. Also certain atmospheric conditions can giverise to reflections. Thus, if the ranging system is designed to respondonly to the first received echo pulse, there is a great probability thatthe distance indicated thereby is not the distance to the intendedobject.

To reduce this uncertainty in the distance measurement it has previouslybeen proposed to provide the receiver of the ranging system withblocking means which prevent certain of the incoming echo pulses fromactu ating the time measuring and recording means of the ranging system.A first such blocking device, called minimum range gate, is adapted toblock the receiver for such echo pulses which arrive within a certainminimum time after the transmission of a pulse from the transmitter, andanother blocking device, called maximum range gate, is adapted to blockthe receiver for such echo pulses which arrive after a certain maximumtime from the transmission of a pulse. These blocking devices arepreferably so designed that the said minimum and maximum times can beadjusted by the operator handling the ranging system. Guided by his ownjudgment the operator makes an estimation of the distance to the targetand the probable error limits of this estimation. With the guidance ofthis estimation the operator sets the minimum range and maximum rangegates. In the following distance measurements only echo pulses fromobjects situated within the predetermined limits will effectivelyactuate the receiving means of the ranging system. The said distancelimits must, however, be rather wide to make certain that the intendedtarget will be located within the limits. Thus, in addition to theintended target there may still be present several other objects whichmay give rise to echo pulses. If only one of these echo pulses, forinstance the first one to arrive, is caused to actuate the indicatingmeans of the ranging system, there is no certainty that the distanceindicated thereby is the distance to the intended target.

To eliminate this uncertainty it has been proposed to provide theranging system with several indicating devices and means for causingthese indicating devices to indicate one distance each of severaldistances represented by several incoming echo pulses. Thus, the rangingsystem records several distances at one shot. (The term shot here meansthe transmission of one pulse from the transmitter of the rangingsystem, for instance a light pulse generated by a laser). By anestimation the operator can then with very large certainty decide whichof the recorded distances is the distance to the intended target. Inorder to make possible the recording of several distances at one shot itis previously known to provide the ranging system with several countersand a distributing device which distributes the incoming echo pulses tothe different counters. However, this arrangement means a considerablecomplication 'and increase of costs of the equipment.

OBJECT AND SUMMARY OF THE INVENTION It is an object of the presentinvention to provide a ranging system and particularly a ranging systemutilizing a laser which affords a considerably simpler way of recordingseveral distances at one shot.

According to the invention the system comprises at least one registerwhich is connected to the counter over a gate which is normally closedand which is momentarily opened in response to at least one receivedreflected pulse, thereby causing the register to record the momentaryposition of the counter at the time of the reception of the reflectedpulse.

If the system comprises several registers these are connected to thecounter over one gate each. In such case a distributor is connected tothe receiver for automatically distributing incoming pulses to differentoutputs which are connected to separate ones of said gates so that therespective gate is opened by a pulse appearing on the correspondingoutput from the distributor, so that the position of the counter at thetime of reception of this pulse is recorded in the register connected tothe gate.

The counter is preferably so designed that the counting result ispresented in binary-coded decimal form and for each decimal digit placeit contains four flipflops which are arranged in known manner torepresent the decimal digit in a binary code, for instance the842l-code. Since the propagation of carry digits in the counter requiresa certain time which is not negligible, the reading of the counter may,however, rend a faulty result if the reading occurs while thepropagation of carry digits is going on. The error becomes particularlylarge if the reading takes place during a count step accompanied bycarry propagation between decades and additionally a plurality of carrypropagations within the most significant decade, as is the case forinstance when the counter steps from 799 to 800.

To .eliminate this source of errors a preferred embodiment of theinvention comprises in addition to the aforementioned counter which inthe following will be named main counter, an auxiliary counter which isstarted by an incoming reflected pulse and thereby prevents furtheroscillations from the oscillator from actuating the main counter andafter a certain time which is sufficiently large to allow the countingelements of the main counter to settle in correspondence to the lastreceived oscillation from the oscillator, transmits a control to thedistributor pulse for opening one of the gates.

The auxiliary counter after having performed a predetermined number ofcounting steps causes the main counter to be actuated again by theoscillations from the oscillator and then causes the main counter tojump over the said predetermined number of counting steps so that themain counter when continuing the counting indicates the number ofoscillations which it would have received if the supply of oscillationshad not been stopped. When the next echo pulse arrives the main counteris stopped again and then indicates a number of received oscillationscorresponding to the time of travel of the last received echo pulse, andthis new indication is recorded in a register.

The control input of one of the gates which connect the main counterwith a register may be connected to a switch by means of which the gatecan be alterna- W tively connected to one of the outputs'from thedistributor or directly to means for producing a pulse in response to areceived echo pulse.

When the gate is connected in accordance with the lastmentionedalternative the corresponding register will successively record theposition of the main counter for each received echo pulse, and after thereception of the last echo pulse there remains in this register arecording corresponding to the distance to that object which caused thereflection of the last received echo pulse. This arrangement may beadvantageous if the number of incoming echo pulses is larger than thenumber of registers. By the said arrangement the distance correspondingto the last received echo pulse will always be recorded.

Another embodiment of the invention comprises only two registers and acomparison device which compares the values indicated by the timemeasuring means with a predetermined reference value and emits differentoutput signals depending on whether the momentary value indicated by themeasuring means is less than or larger than the said reference value,and a switching device which makes one or the other of said registerseffective on the reception of reflected pulses. Said switching device iscontrolled by the output signal from the comparison device so that oneregister is made effective as long as the values indicated by themeasuring means are less than said reference value and the otherregister is made effective when upon reception of the reflected pulsesthe value indicated by the measuring means for the first time exceedssaid reference value.

The said reference value can for instance be a value estimated by theoperator or a value obtained in a preceding measuring procedure.

Thus, in this embodiment a remaining recording is obtained only of thedistances to those two objects whose distances from the place ofmeasurement is next less than and next larger than the distancerepresented by the said reference value. The operator selects from thesetwo recorded values that which seems most probable.

In this embodiment also the registers are connected to the counter overgates which are normally closed. The switching device is adapted to openone or the other of the gates on the reception of a reflected pulsedepending on whether the value indicated by the counter on the receptionof a pulse is less than or larger than the reference value.

DRAWINGS AND DETAILED DESCRIPTION In the following the invention will bedescribed more in particular with reference to the accompanyingdrawings.

FIG. 1 shows in the form of a block circuit diagram an embodiment of theinvention.

FIG. 2 shows another embodiment of the invention.

In the figures like reference numerals designate like parts.

Each of the embodiments shown in the drawings may form part of acomplete ranging system operating with light pulses generated by a lasertransmitter. The generated light pulse is reflected by one or moreobjects situated in the direction of transmission, and the reflectedpulses (the echo pulses) are received and converted to electric pulsesand amplified in a receiving system. The laser transmitter and the meansfor the reception and amplification of the echo pulses are assumed to bedesigned in some known manner and will not be described in detail.

In FIG. 1 numeral 1 designates a terminal which is connected to atransmitter T which includes a laser transmitter and means whichgenerate a pulse, in the following named starting pulse, at the sametime as a light pulse is transmitted from the laser transmitter. Theterminal 2 is connected to the receiving means R so that the incomingand amplified echo pulses are supplied to this terminal.

Over terminal 2 the echo pulses are first transferred to a gate 3 whichtogether with a device 4 forms a minimum range gate. As mentioned above,the purpose of this device is to prevent echo pulses which arrive withina certain minimum time after the transmission of a pulse from actuatingthe counting and indicating means. The device 4 may for instance consistof a time delay device having adjustable time delay which under theaction of a start pulse over terminal 1 after a certain time delivers anopening pulse to gate 3.

The means for measuring the time intervals between the start pulse andthe echo pulses comprises a clock oscillator 5 and a counter 7, in thefollowing named the main counter. The output of the oscillator 5 isconnected to the input of the main counter 7 over a gate 6 which isnormally closed and is opened under the action of a start pulse fromterminal 1. When gate 6 is opened the main counter 7 receivesoscillations from oscillator S and counts these oscillations.

The frequency of the oscillations generated by oscillator 5 isdetermined by the desired measuring accuracy. If for instance ameasuring accuracy of 10 meters is desired, the frequency of oscillator5 should be 15 Mc/s.

The main counter 7 presents the counting result in binary coded decimalform and for this purpose it contains four flip-flops for each decimaldigit place. The capacity of the counter is adapted to the maximumdistance which is to be measured by the ranging system. If the counter 7comprises three decimal digit places it can count to 999, and if eachcount corresponds to a distance interval of 10 meters, the system canmeasure distances up to 9990 meters. The counter 7 is so designed thatafter having performed the maximum possible number of counting steps itresets itself and simultaneously emits a pulse to gate 6 which isthereby closed. For reading the state of the main counter 7 there areprovided a readout line 10 which is branched in three branches 11, 12and 13 which are connected over gates 21, 22 and 23 respectively toregisters 31, 32 and 33 respectively. The line 10 and each of thebranches 1]., 12 and 13 contain a number of conductors corresponding tothe number of flip-flops in the counter 7, and in the counter theconductors are connected in known manner to each one flip-flop so thatone or the other of two potentials appear on each conductor according tothe state of the flip-flop.

The registers 31, 32 and 33 are connected over suitable decoders (notshown) to indicating means (not shown) for indicating the values storedin the respective registers. These indicating means may for instanceconsist of a digital display unit.

The control inputs of gates 21, 22 and 23 are connected to each oneoutput 41, 42, and 43 respectively of a distributing device 40. The echopulses which pass through the gate 3 are forwarded over line 8 to thedistributing device 40 which is designed in known manner so that'thefirst pulse to arrive is passed to output 41, the second pulse to output42 and the third pulse to output 43. These pulses open the respectivegates 21, 22 and 23 so that the setting of the main counter is read outupon the arrival of the respective echo pulses, and the read-out valuesare stored in the registers 31, 32 and 33 respectively.

As mentioned above it may occur that an echo pulse arrives while thecounter 7 is in a stage of transition from one value to the next value.If the read-out of the counter occurs while this transition is going onthe read-out value may be entirely erroneous. In order to eliminate thissource of error the system is provided with an additional counter,namely the auxiliary counter 14 which has a considerably less countingcapacity than the main counter 7. The auxiliary counter 14 is adapted toreceive the incoming echo pulses over line 9. An incoming echo pulsestarts the auxiliary counter 14 so that it counts oscillations fed to itover line 15 from oscillator 5. At the same time the auxiliary counter14 transmits a pulse over line 16 which closes gate 6. Hereby thefeeding of oscillations from oscilla tor S to the main counter 7 isinterrupted, and when the counting members of the main counter 7 havesettled in correspondence to the last received oscillation, the maincounter remains in the set position. In the meanwhile the auxiliarycounter 14 counts oscillations which arrive from oscillator 5 over line15. When the auxiliary counter 14 has counted a predetermined number ofoscillations it transmits over line 14 a pulse to the distributingdevice 40. The echo pulse which caused the auxiliary counter 14 tostart, is also applied over line 8 to the distributing device 40 which,however, in this case is so designed that the echo pulse arriving overline 8 does not immediately cause any output pulse to appear on any ofthe outputs 41, 42 or 43. Only when a pulse from the auxiliary counter14 arrives over line 18 to the distributing device 40, one of theoutputs 41-43 is activated and the gate 21, 22 or 23 connected to thatoutput is opened during a short period of time whereby the setting ofthe main counter 7 is recorded in the register 31, 32 or 33 associatedwith that gate.

After the setting of the main counter 7 has been recorded in thismanner, the main counter 7 is started again, for instance by theauxiliary counter 14 upon the next counting step transmitting an openingpulse over line 16 to gate 6. At the same time the auxiliary counter 14actuates the main counter 7 so that the latter when again receivingoscillations from oscillator 5 jumps over a number of stepscorresponding to the number of oscillations counted by the auxiliarycounter 14 during the time when the feeding of oscillations to the maincounter 7 was interrupted. This jump can be effected in some knownmanner under the control of the auxiliary counter 14 over line 17. Thus,when the main counter 7 is continuing the counting the number indicatedby the main counter 7 will all the time be equal to the total number ofoscillations generated by oscillator 5 from'the time when the startingpulse arrived over terminal 1. When the auxiliary counter 14 hasperformed the functions now described it is automatically reset to thezero state. When the next echo pulse arrives the described process isrepeated and the setting of the main counter 7 after the arrival of thenext echo pulse is recorded in the next register.

The predetermined number of oscillations to be counted by the auxiliarycounter 14 before it causes the setting of the main counter to berecorded is deter mined by the maximum transition time required by themain counter 7 for the transition from one step to the next one. Thistransition time may in the most unfavorable case amount to nanoseconds.If the ranging system is designed for a measuring accuracy of 10 meters,the oscillations from the oscillator 5 should have a period of about 67nanoseconds which is less than the said maximum transition time for themain counter 7 but larger than one half of this transition time. In thiscase the auxiliary counter 14 should therefore be adapted to count twooscillations before it causes the setting of the main counter 7 to beread out and recorded in one of the registers 31-33, and when the maincounter 7 is started again it should be caused to skip two steps.

The device described above and shown in FIG. 1 is designed for recordingthree distances. However, it will be understood that the device can bedesigned for recording a larger number of distances by increasing thenumber of registers and the associated gates and providing thedistributing device 40 with more outputs.

However, in a distance measuring operation it may occur that the numberof objects situated at different distances from the ranging system andgiving rise to echo pulses larger than the number of recording elementsin the ranging system so that the distances to all these objects cannotbe recorded. If the time measuring and recording means operate in themanner described above and the number of registers is for instance threeas in the shown embodiment, the distances to the three nearest objectswill be recorded while the distances to additional objects which giverise to echo pulses will not be recorded. However, it may of interest inthis case to know the distance to the most remote object, that is thedistance to the object which reflects the last received echo pulse. Thisis possible in the embodiment shown in FIG. 1 by means of a manuallyoperable switch 20 which alternatively connects the control input ofgate 23 to the output 43 of the distributing device 40 or to a branch 19of the line 18 from the auxiliary counter 14. When the switch 20 is inthe position indicated by a full line on the drawing and assuming thatthe outputs 41, 42 and 43 of the distributing device 40 is activated inthe said order in response to the incoming echo pulses, the third echopulse will cause the gate 23 to be opened so that the correspondingdistance is recorded in the register 33. If the switch 20 is in theposition indicated by the dotted line, the gate 23 will receive anopening pulse from the auxiliary counter 14 for each received echopulse, and the distances to all objects giving rise to echo pulses willbe recorded in turn in register 33. For each new recording in theregister the previous recording is cancelled. After the measuringprocedure has been finished a recording corresponding to the distance tothe most remote object will therefore remain in register 33, whileregisters 31 and 32 indicate the distances to the two nearest objects.

In some cases it is of interest only to know the distance to the mostremote object. In such case the distance may comprise only one register(33 in FIG. 1), and the associated gate (23) may have its control inputpermanently connected to the mans (e.g. the auxiliary counter 14) forproducing opening pulses in response to the received echo pulses. Inthis case the distributing device 40 may be omitted.

The distributing device 40, if provided, may be combined with means forcounting the total number of incoming echo pulses or the number of echopulses other than those which cause a recording.

FIG. 2 shows an embodiment in which a recording is obtained only of thedistances to two objects whose distances from the measuring site isnearest less than and nearest larger than a certain predeterminedreference value.

In FIG. 2 numerals 1-7 designate the same parts as in FIG. 1.

For the read-out of the counter 7 there are provided three read-outlines 49, 50 and 51. Line 49 is connected over gate 21 to a firstregister 31, and line 50 is connected over gate 22 to a second register32. The registers 31 and 32 are connected over suitable decoding means(not shown) to indicating means (not shown) for indicating the numbersstored in the respective registers.

The gates 21 and 22 are normally closed. Their control inputs areconnected to outputs 54 and 55 respectively of a switching device 52.The echo pulses which arrive over terminal 2 and pass through gate 3 aretransferred over line 53 to this switching device 52. The switchingdevice 52 directs the received pulses either over output 54 to gate 21or over output 55 to gate 22 in dependence of a control signal which isapplied over line 58 to the switching device 52 from a comparison device56. When gate 21 or gate 22 receives a pulse the gate is opened so thatthe setting of the counter 7 at this time is recorded in the register 31or 32 respectively.

The comparison device 56 is connected to the counter 7 over the read-outline 51 and thus receives a continuous information on the setting of thecounter. A reference value corresponding to an estimated distance is fedinto the comparison device 56 over lines 57. The reference value isrepresented in some known manner by a suitable combination of signals onlines 57.

In the comparison device 56 the setting of counter 7 is compared withthe said reference value. As long as the value indicated by counter 7 isless than the reference value the comparison device 56 delivers anoutput signal ofa first kind over line 58 to the switching device 52which is controlled by this output signal so that the echo pulsesarriving over line 53 are directed to gate 21 over line 54.. Therefore,as long as the counter 7 indicates values below the reference value thesetting of the counter 7 for each incoming echo pulse will be recordedin register 31. For each new recording the previous recording iscancelled.

When the counter 7 indicates values exceeding the reference value thecomparison device 56 delivers an output signal of a second kind whichactuates the switching device 52 so that the next echo pulse to arriveis directed over line 55 to gate 22 so that the gate is opened and thesetting of the counter 7 at the time of arrival of this pulse isrecorded in register 32. After that the register 32 is immediatelyblocked so that this recording remains even if additional echo pulsesshould arrive. This blocking can be effected by a suitable design ofregister 32 or by designing the switching device 52 in such manner thatit only allows one single pulse to be applied to gate 22, when theoutput signal from the comparison device 56 is of the said second kind.The output signals from the comparison device 56 may consist ofdifferent potentials on line 58, and one of these potentials may even bezero potential.

The various parts included in the device according to the invention suchas the clock oscillator, counters, gates, switches and registers are alldesigned in a manner well known in the art, and therefore they have notbeen described in detail. The parts are of course of the electronic typeand are preferably equipped with semiconductor components.

The embodiments described and shown are given by way of examples onlyand can be modified in many ways within the scope of the invention.

What is claimed is:

1. Ranging system comprising transmitting means for transmitting ashort-duration pulse of electromagnetic radiation, receiving means forreceiving reflected pulses, and time measuring means for measuring thetime interval between the transmission of a pulse and the reception ofone or more reflected pulses, said time measuring means including anoscillator for generating electric oscillations and a first counter forcounting the oscillations generated by the oscillator from the time oftransmission of a pulse, characterized in that it further comprises atleast one register (31,32,33), connecting means (11,12,13) connectingsaid first counter (7) with the said register, a normally closed gate(21,22,23) in said connecting means, gate opening means (40,4l,42,43)for momentarily opening said gate in response to a control pulse afterat least one received reflected pulse, thereby causing the said registerto record the momentary position of the said counter at the time of thereception of the reflected pulse, and a second counter (14) which isstarted in response to an incoming reflected pulse and then preventsfurther oscillations from the oscillator (5) to act upon said firstcounter (7 and after a time which is sufficiently long to allow thefirst counter to settle in response to the last received oscillationfrom the oscillator, transmits a control pulse to said gate openingmeans for opening one of said gates.

2 Ranging system as claimed in claim 1, characterized in that itcomprises a plurality of registers (31,32,33) which are connected .tosaid first counter (7 over separate gates (21,22,23), and that adistributing device (40) is connected to the said receiver means ofreflected pulses, said distributing device being adapted to distributeautomatically incoming pulses to different outputs (41,42,43) which areconnected to separate ones of said gates (21,22,23), so that each gateis opened by a pulse appearing on the corresponding output from saiddistributing device so as to cause the time for the appearing of thispulse to be recorded in the register associated with the gate.

3. Ranging system as claimed in claim 1, characterized in that saidsecond counter (14) after a predetermined number of counting stepscauses the first counter (7) to respond again to oscillations from saidoscillator, thereby causing the counter to continue the counting and indoing so jumping over said predetermined number of counting steps.

4. Ranging system as claimed in claim 1, characterized in that thecontrol input of at least one (23) of said gates is directly connectableto means (14) for producing opening pulses in response to receivedreflected pulses.

5. Ranging system as claimed in claim 4, characterized in that thecontrol input of at least one (23) of said gates is connected to aswitch (20) for alternatively connecting said control input to one (43)of the outputs from said distributing means (40) or directly to saidmeans (14) for producing opening pulses in response to receivedreflected pulses.

6. Ranging system comprising transmitting means for transmitting ashort-duration pulse of electromagnetic radiation, receiving means forreceiving reflected pulses, and time measuring means for measuring thetime interval between the transmission of a pulse and the reception ofat least one reflected pulse, said time measuring means including anoscillator for generating electric oscillations and counting means forcounting the oscillations generated by said oscillator from the time oftransmission of a pulse, a first register and a sec- 0nd register forrecording the values indicated by said time measuring means, acomparison device for comparing the values indicated by said timemeasuring means with a predetermined reference value and to producedifferent output signals depending on whether the value indicated bysaid time measuring means is less or greater than said reference value,and switching means for making one or the other of said registerseffective to record the position of said counting means upon thereception of reflected pulses, said switching means being controlled bythe output signal from said comparison device so that one register ismade effective as long as the values indicated by said measuring meansare less than said reference value and the other register is madeeffective when, upon the reception of the reflected pulses, the valueindicated by said time measuring means for the first time exceeds saidreference value.

7. Ranging system as claimed in claim 6, wherein each of said registersis connected to said time measuring means via a gate which is normallyclosed, and said switching means, upon the reception of a reflectedpulse, opens one or the other of said gates depending on whether thevalue indicated by said time measuring means upon the reception of thatpulse is less or greater than said reference value.

1. Ranging system comprising transmitting means for transmitting ashort-duration pulse of electromagnetic radiation, receiving means forreceiving reflected pulses, and time measuring means for measuring thetime interval between the transmission of a pulse and the reception ofone or more reflected pulses, said time measuring means including anoscillator for generating electric oscillations and a first counter forcounting the oscillations generated by the oscillator from the time oftransmission of a pulse, characterized in that it further comprises atleast one register (31,32,33), connecting means (11,12,13) connectingsaid first counter (7) with the said register, a normally closed gate(21,22,23) in said connecting means, gate opening means (40,41,42,43)for momentarily opening said gate in response to a control pulse afterat least one received reflected pulse, thereby causing the said registerto record the momentary position of the said counter at the time of thereception of the reflected pulse, and a second counter (14) which isstarted in response to an incoming reflected pulse and then preventsfurther oscillations from the oscillator (5) to act upon said firstcounter (7) and after a time which is sufficiently long to allow thefirst counter to settle in response to the last received oscillationfrom the oscillator, transmits a control pulse to said gate openingmeans for opening one of said gates.
 2. Ranging system as claimed inclaim 1, characterized in that it coMprises a plurality of registers(31,32,33) which are connected to said first counter (7) over separategates (21,22, 23), and that a distributing device (40) is connected tothe said receiver means of reflected pulses, said distributing devicebeing adapted to distribute automatically incoming pulses to differentoutputs (41,42,43) which are connected to separate ones of said gates(21,22,23), so that each gate is opened by a pulse appearing on thecorresponding output from said distributing device so as to cause thetime for the appearing of this pulse to be recorded in the registerassociated with the gate.
 3. Ranging system as claimed in claim 1,characterized in that said second counter (14) after a predeterminednumber of counting steps causes the first counter (7) to respond againto oscillations from said oscillator, thereby causing the counter tocontinue the counting and in doing so jumping over said predeterminednumber of counting steps.
 4. Ranging system as claimed in claim 1,characterized in that the control input of at least one (23) of saidgates is directly connectable to means (14) for producing opening pulsesin response to received reflected pulses.
 5. Ranging system as claimedin claim 4, characterized in that the control input of at least one (23)of said gates is connected to a switch (20) for alternatively connectingsaid control input to one (43) of the outputs from said distributingmeans (40) or directly to said means (14) for producing opening pulsesin response to received reflected pulses.
 6. Ranging system comprisingtransmitting means for transmitting a short-duration pulse ofelectromagnetic radiation, receiving means for receiving reflectedpulses, and time measuring means for measuring the time interval betweenthe transmission of a pulse and the reception of at least one reflectedpulse, said time measuring means including an oscillator for generatingelectric oscillations and counting means for counting the oscillationsgenerated by said oscillator from the time of transmission of a pulse, afirst register and a second register for recording the values indicatedby said time measuring means, a comparison device for comparing thevalues indicated by said time measuring means with a predeterminedreference value and to produce different output signals depending onwhether the value indicated by said time measuring means is less orgreater than said reference value, and switching means for making one orthe other of said registers effective to record the position of saidcounting means upon the reception of reflected pulses, said switchingmeans being controlled by the output signal from said comparison deviceso that one register is made effective as long as the values indicatedby said measuring means are less than said reference value and the otherregister is made effective when, upon the reception of the reflectedpulses, the value indicated by said time measuring means for the firsttime exceeds said reference value.
 7. Ranging system as claimed in claim6, wherein each of said registers is connected to said time measuringmeans via a gate which is normally closed, and said switching means,upon the reception of a reflected pulse, opens one or the other of saidgates depending on whether the value indicated by said time measuringmeans upon the reception of that pulse is less or greater than saidreference value.